ART: Improve IsValidMemberName

Force inlining of the fast-path IsValidPartOfMemberNameUtf8.

perf stat of dexdump -c of a large app,
  before:
     4,867,415,428      instructions:u
     1,089,412,280      branches:u
        26,880,249      branch-misses:u
  after:
     4,425,845,396      instructions:u
       978,781,201      branches:u
        26,585,670      branch-misses:u

Test: m test-art-host
Change-Id: I5b96a9fa4c3f2ec70126c4931e01f30b5e8a1172